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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

324

BENZOLES

The copper vessel rests on a circular aperture, 50 mm. in diameter, in
a piece of asbestos card E, supported on an oven closed at the sides and
provided at its upper part with four 10 mm. holes to allow of the circulation
of the air. The heating is effected by a Bunsen burner of 7 mm. aperture.

The lateral tube of the dephlegmator is connected with a condenser D,
800 mm. long, inclined so that the top end is 100 mm. higher than the free end.

With this apparatus 100 c.c. of the liquid are distilled in such a manner
that 5 c.c. distil over per minute (2 drops per second), fractions passing
over at different temperatures (up to 100°, 120°, 145°, 160°, 175°, ig0°,
according to the different types of benzole) being collected in a graduated
cylinder and measured.

For exact determinations it is necessary to take account of the atmospheric
pressure, bearing in mind that for pressures between 720 and 780 mm. the
percentages given by the distillation should be diminished by 0-033 for 90 °/
benzoles and by 0-077 for 5°% benzoles for each millimetre of pressure below
the normal pressure of 760 mm. and increased by the same amounts for each
millimetre above 760 mm.

3. Determinations of the Separate Hydrocarbons.—To separate
and estimate approximately the different hydrocarbons contained in com-
mercial benzole, the latter must be fractionally
distilled in a manner rather different from that
just described, a moderately large amount of sub-
stance being treated in an apparatus furnished
with an efficient dephlegmator. Use is generally
made of a copper vessel of the form and dimensions
indicated (in millimetres) in Fig, 38, a six-bulb
Le Bel-Henninger fractionator, 60 cm. long, being-
fitted to it. The fractionator is provided with a.
thermometer and joined to a condenser, and i kilo
of the product is distilled at the same rate as in 2
(above), the different fractions being collected in
tared receivers, which are subsequently reweighed.
The separation of the different hydrocarbons
may be effected by further fractional distillations,
regard being paid to the boiling points, which are
as follows : benzole, 80-81°; toluole, 110-111° ;
xyloles, 138-142° (o-xylene, 142° ; m-xylene, 139-
140°; ^-xylene, 138-139°); ethylbenzene, 137°; trimethylbenzenes
163-175°-

The fractionation of the different commercial products is carried out on the
basis of the following temperature limits :

, — ...... JlOn

oU.. ....-.





FIG. 38

First fraction

Benzole

Intermediate fraction   .

Toluole

Xyloles       .      • .

Higher homologues, etc.

Benzole      Pure commercial
(50% and 90%):      Benzole.

up to 79°
79-85
85-105
105-115

residue

up to 79°
79-81

residue

Toluole.
up to iogc
109-110-5
residue

Xyloles.

up to  135°

residue when filtered through paper, the solution thus obtained should